1. Field of the Invention
The present invention relates to electrical insulators having semi-conducting glaze, and more particularly relates to an electrical insulator having a semi-conducting glaze, which comprises an insulating body with its surface coated with a semi-conducting glaze, metal fittings and a cement composition filled between the insulating body and the metal fittings.
2. Description of the Prior Art
Electrical insulators having semi-conducting glaze have a unique pollution resistance and have recently been adopted in power transmission lines. In the electrical insulators, a semi-conducting glaze layer formed on the surface of an insulating body must be electrically connected to metal fittings to cause very weak electric current to flow through the semi-conducting glaze layer. Various connecting methods have hitherto been proposed for this purpose. In a typical connecting method, a metal is hot sprayed on the glaze, cement and metal fittings to form metal film layers thereon and to form electric current paths. However, this method has drawbacks, in cost, mass productivity, liability and the like. In another typical connecting method, conductive cement is filled between an insulating body and metal fittings. However, conventional conductive cement has a low mechanical strength, so that this method has the drawback that an electrical insulator as a whole must be large in size. Therefore, the development of new conductive cement having a high mechanical strength has been eagerly desired. The electrical conductivity of conventional conductive cement compositions is given by conductive materials, such as carbon black, graphite, carbon fiber and the like, mixed with cement. Among these conductive materials, graphite itself has an electric resistance of about 1/10 - 1/100 times lower than that of carbon black, but a cement composition having a desired electric conductivity cannot be obtained unless a large amount of graphite is added to cement, and therefore the mechanical strength of the resulting cement composition is decreased due to the presence of a large amount of graphite in the cement composition. Although carbon fiber itself has an electric resistance of about 1/10 - 1/100 times lower than that of carbon black, when carbon fiber is mixed with cement, the resulting cement composition is low in the mechanical strength, is poor in workability due to its very low fluidity during use, and is remarkably uneven in the electric resistance. Moreover, when the cement composition is used under a high current density, the electric circuit is thermally broken and the cement composition is apt to lose its electric conductivity. In order to obviate these drawbacks, conductive cement compositions to be used in electrical insulators, which require a high strength, have been proposed, for example, in U.S. Pats. Nos. 3,836,705 and 3,941,918 and in British Pat. No. 1,398,306. In the above described U.S. Pat. No. 3,941,918, a conductive cement composition obtained by mixing Portland cement with carbon black is used. However, the U.S. Patent discloses that the use of carbon black alone is insufficient for giving a satisfactorily high mechanical strength to cement, and 0.5-2.0 parts by weight of graphite fiber is further added to the cement composition to obtain a cement composition having a desired mechanical strength required in electrical insulators. However, the resulting cement composition has a compression strength of at most about 633-703 Kg/cm.sup.2, and moreover the cement composition is remarkably uneven in the electric resistance due to the presence of the graphite fiber and is very poor in the workability. In the "Electrically conductive cement" disclosed in the above described British Pat. No. 1,398,306, sulfur cement is mixed with fine graphite. However, the resulting conductive cement also has a low mechanical strength, and therefore the cement can be applied to a column-shaped insulator, particularly the so-called multi-cone type insulator, which does not require so high mechanical strength. However, when the cement is used in a disk-shaped suspension insulator or the like, which requires a high mechanical strength, the electrical insulator must be designed into a particular shape in order to compensate for the reduction in the mechanical strength of the cement and as a result it is obliged to provide a large size electrical insulator as a whole.
As described above, it has hitherto been a general recognition in the technical field of the present invention that carbon black cannot be used alone in order to give adequate electric conductivity to a cement composition to be filled between the insulating body of an electrical insulator and the metal fittings thereof. One of the reasons of such recognition by those skilled in the art is as follows. Carbon black is finely granular and porous and has a large specific surface area. Therefore, in order to obtain a cement composition containing carbon black and further having high fluidity and workability, the water-cement ratio must be fairly high. However, when the water-cement ratio is high, the mechanical strength of the cement composition is considerably decreased and the shrinkage thereof becomes high. Therefore, the cement composition loses performances necessary for fillings between the insulating body and the metal fittings in an electrical insulator.
The inventor has investigated the parameters, which influence the electric conductivity of hardened cement composition containing carbon black, and succeeded to overcome the general recognition by those skilled in the art relating to the behavior and faculty of carbon black in a cement composition, thereby eliminating the above described drawbacks of conventional electrical insulators.